1. Field of the Invention
The present invention relates in general to wellhead seals and, in particular, to a damage tolerant casing hanger seal.
2. Brief Description of Related Art
During creation of a wellhead, drilling operations often cause damage to casing elements of the well inserted into the wellbore prior to the switch from drilling to production. In particular, casing hangers and high pressure wellhead housing can be damaged with scratches and gouges that range from minor scratches, such as a few thousandths deep, to major scratches, as much 0.1″ deep. In order to seal the wellbore, seals must be constructed of compliant material that can extrude and fill the scratches and gouges. Currently, seals consisting of elastomer seal elements are used to seal the wellbore annulus between the casing elements.
Unfortunately, elastomer seal elements do not meet the current needs of well drilling and production. For example, many seals are used in conditions where the seal is subjected to extreme cold, extreme heat, and/or cycles between the two extremes. In these situations, the elastomer seals fatigue and fail prior to the end of the seal's desired field life. Current industry standards allow for a ninety day seal field life. However, industry desires a seal that can last the expected life of the well, approximately twenty years. In addition, the life cycle of the well may include start up and shut down of the well, pressure testing of well elements, and the like. These life cycle activities increase the number of extreme stress and temperature cycles to which elastomer seals are exposed. The life cycle activities cause elastomer seals to fatigue and fail well before the twenty year desired field life requirement of wellhead seal systems. Therefore, there is a need for wellhead seals that can withstand extreme temperatures, extreme temperature cycling, and varying life cycle operations of the well up to an expected life cycle of twenty years.
In addition, elastomer seal elements may experience explosive decompression. Explosive decompression occurs when high pressures surrounding an eleastomer sealing element force the elastomer seal to absorb gases from the surrounding environment. When pressures surrounding the eastomer seal element drop, the gases absorbed into the elastomer seal element at higher pressures rapidly escape the elastomer seal element. The rapid escape of gases causes tearing and destruction of the elastomer seal element. Therefore, there is a need for wellhead seals that are not subject to failure by explosive decompression during high pressure loading and unloading cycles.
Many wellhead seals use elements that seal the annulus between wellhead members by means of an interference seal. Interference seals use sealing members with a slightly larger width than the annular space to be sealed. Interference seals force the sealing members into the sealing area to prevent passage of fluid or other materials. Because these interference seals are larger than the annular space to be sealed, use of an interference seal often causes damage to the annular space to be sealed and the interference seal itself. This hinders drilling and operation of the well and leads to early failure of the seal. Therefore, there is a need for wellhead seals that will not damage the wellhead casing elements during insertion and energizing.
Many wellhead seal assemblies used to seal an annulus between wellhead casing elements are not retrievable. Once put in place and energized, the seal cannot be removed. It is in the wellhead until the seal fails. This can hinder drilling and operation of the well where the seal may interfere with running tools and the like. In addition, during emergency situations, standard seals cannot be retrieved; thus, once a seal is used in an emergency situation, it must remain in the wellbore. This prevents use of subsequent better seals or the passage of other equipment or tools through the space. Therefore, there is a need for wellhead seals that are retrievable.